1. Field of the Invention
The present invention relates to compositions in the form of a silicone oil emulsion, which are intended to be applied to curing bladders and/or to pneumatic or semi-pneumatic tires, to make molding/mold stripping easier during the manufacture of tires.
The invention relates in particular to an application as a lubricating composition particularly appropriate for the lubrication of curing bladders used during the shaping and curing of pneumatic or semi-pneumatic tires.
The invention also relates to the curing bladders coated with a lubricating composition and/or an adhesion primer according to the invention and to the pneumatic or semi-pneumatic tires coated with said lubricating composition.
According to two of its other aspects, the invention relates to a method of preparing lubricating compositions of the invention and to the use of said lubricating compositions for the lubrication of curing bladders.
2. Description of Related Art
Rubber tires for vehicles are conventionally manufactured by molding and curing a green, or uncured and unshaped, tire in a molding press in which the green tire is pressed toward the outside against the surface of a mold by means of a bladder that can be inflated by an internal fluid. By this method, the green (i.e. uncured) tire is shaped against the external surface of the mold, which defines the tread pattern of the tire and the configuration of the sidewalls. By applying heat, the tire is cured. In general, the bladder is inflated by the internal pressure provided by a fluid, such as a hot gas, hot water and/or steam, which also participates in the transfer of heat for the curing operation. The tire is then left to cool down slightly in the mold, this cooling being sometimes promoted by introducing cold or cooler water into the bladder. The mold is then opened, the bladder is deflated, by releasing the pressure of the internal fluid, and the tire is removed from the tire mold. This use of tire-curing bladders is well known in the art.
It is recognized that there is substantial relative movement between the external contact surface of the bladder and the internal surface of the tire during the bladder inflation phase before complete curing of the tire. Likewise, there is also considerable relative movement between the external contact surface of the bladder and the cured internal surface of the tire, after the tire has been molded and cured, during deflation of the bladder and its extraction from the tire.
Unless adequate lubrication is provided between the bladder and the internal surface of the tire, the bladder generally has a tendency to buckle, resulting in a misshaping of the tire in the mold and also excessive wear and roughening of the bladder surface itself. The bladder surface also tends to stick to the internal surface of the tire after the tire has been cured and during that part of the tire curing cycle when the bladder is being deflated. Furthermore, air bubbles may be trapped between the surface of the bladder and that of the tire, and promote the appearance of tire curing defects resulting from inadequate heat transfer.
For this reason, the external surface of the bladder or the internal surface of the green or uncured tire is coated with a suitable lubricant, sometimes referred to by the name “lining cement”.
Many lubricating compositions have been proposed for this purpose in the art.
In particular, lubricating compositions described in Patent Application FR 2 494 294 are known, which contain, as principal constituents, a reactive polydimethylsiloxane preferably having terminal hydroxyl groups, a crosslinking agent preferably comprising Si—H functional groups, and optionally a polycondensation catalyst.
U.S. Pat. No. 3,872,038 describes an internal mold release agent for tires, based on a silicone emulsion and a filler, for example mica, to ensure air can escape effectively. Instead of a reactive system, a silicone gum is used to keep the filler on the surface of the rubber after application. This formulation allows the air trapped between the bladder and the green tire to escape, but does not allow several mold stripping operations to be carried out.
Usually, the inflatable bladders, before being coated on their external surface (that which will come into contact with the tire) with a lubricating composition, may undergo a pretreatment consisting in applying a uniform layer of a composition called a “primer” or an “adhesion primer”.